Although the pistons used in reciprocating type internal combustion engines are nominally "cylindrical," they have in practice been contoured or profiled to take into account the differential thermal expansion that takes place in normal operating conditions. The head of a piston becomes heated to a significantly higher temperature than the skirts, and there is thus a temperature gradient along the length of the piston which means that the piston "grows" to different extents in different axial parts. Moreover, there are differentials in the growth of the piston at different circumferential locations. Accordingly, it is common to either grind or machine a piston to a selected circumferential profile and a selected axial profile to provide desired profiles after the piston grows due to thermal expansion in operation.
By far the most common way of forming the desired axial and circumferential profiles on pistons is by cam-grinding. Cam-grinding involves the use of a grinding stone which is dressed to an axial profile conforming to the desired axial profile to be put on the piston. The desired circumferential profile is provided by moving the piston toward and away from the grinding stone (or vice versa) in coordination with the rotation of the piston, the piston being translated toward and away from the stone (or vice versa) by a cam having a circumferential profile that matches the desired circumferential profile formed on the piston (hence, the name "cam-grinding"). Cam-grinding is an expensive operation in terms of labor time and tooling costs, and it is difficult to control tolerances because the stone wears at different rates in different places. Good control over tolerances requires frequent dressing of the stone.
A small proportion of the total number of pistons produced are finished to the desired axial and circumferential profiles by machining. Most of the machining techniques involve the use of a master cam having circumferential and axial profiles matching the desired profile to be machined on the piston and rotated in one spindle. The workpiece rotates in another spindle at the same speed, and a cutting tool in guided under the control of the stylus that follows the master cam, either by a direct mechanical linkage between the stylus and the tool or by a hydraulic tracing system that is under the control of the cam. Machine profiling is also expensive, slow, and difficult to control with precision.